The present invention relates to a vertebral osteosynthesis device that can be used simply to brace a spine that requires it, for example following accidental fracture, or to straighten and brace a deviated spine, as in the case of scoliosis or kyphosis.
As is known, such a device can comprise two rigid and substantially parallel support rods disposed one on each side of the spinous processes of the vertebrae, on which bone-anchoring elements may be mounted. The bone anchoring elements can be constituted either by hooks designed to fit around the spinous processes or by screws designed to be supported by the pedicles of the vertebrae.
Implantation of a device of this kind requires precise shaping of the rods to adapt them to the curves in one or two planes of the spine, whether these are natural curves or curves resulting from kyphosis or scoliosis, and requires a perfect link to be achieved between the rods and the anchoring elements. The link must be capable of withstanding the repeated stresses over time to which the various elements are subjected by the patient's movements.
International patent application No. WO 91/01691 and European Patent Application No. 0,346,521 describe more specifically anchoring elements comprising, two lateral walls projecting from the side opposite their hooking elements, substantially parallel and flexible, delimiting between them a channel, the width of which is substantially equal to the diameter of the rods, and, a gripping element able to engage the lateral walls and bring them closer together during said engagement. The rods are introduced into the channels and then the gripping elements are put in place. The radial grip exerted on the rod by deformation of said walls and the substantial contact area between the rod and the walls ensure that the hooking elements remain integral with the rod.
The aforementioned shaping of the rods is not easy with these devices because, as long as the rod is not held between flexible legs, it is difficult to check that the shape conferred on the rods is correct or whether this shape must be reworked. Moreover, the strength over time of the anchoring element/rod linkage is uncertain because the attachment of the gripping elements depends on the flexure of the legs.
In addition, the gripping elements have the disadvantages of being difficult to set in place and tighten, and of not allowing the practitioner to be fully in control of the grip on the rod that they produce. The gripping action of the rod is made difficult by the deformation of the walls. Moreover, access to the gripping elements is somewhat limited and is problematical when the gripping elements are constituted by nuts that can be screwed to the ends of the walls because the possible play of the gripping wrench is particularly small in view of the size of the incision and the presence of various instruments.
Moreover, in certain cases it may be useful to achieve a grip that is just sufficient temporarily to immobilize the rods relative to the hooking elements but without locking them permanently together with a view, in particular, to subsequent verification of the curvature of the rods.